1. Technical Field of the Invention
The present invention pertains in general to methods for determining the location of a Global Positioning System (GPS) receiver and, more particularly, to a method for determining the location of a GPS receiver when auxiliary information used for determining the location of the GPS receiver is calculated at a different time than when range measurements are made by the GPS receiver.
2. Description of Related Art
It is desirable, and likely to be mandatory in the near future, that cellular telephone systems be equipped to determine the geographical location of cellular telephones operating within the cellular telephone system. To meet this requirement it has been proposed that cellular telephones be equipped with Global Positioning System (GPS) receivers for determining the location of the cellular telephone. GPS receivers, however, are expensive, increase the cellular telephone size, and consume limited battery power available to the cellular telephone. Furthermore, GPS receivers do not function well within buildings or in other areas where GPS satellite transmissions are weakened due to an obstruction, fading, reflecting, or the like.
It is commonly known that GPS receivers can be made smaller, less expensive, and more energy efficient by eliminating certain GPS receiver functionality used to obtain auxiliary information normally obtained through the demodulation of GPS satellite signals. Instead of demodulating the GPS satellite signals, an alternative means is used to provide the GPS receiver with the needed auxiliary information. This auxiliary information includes various information such as a list of GPS satellites currently in view of the GPS receiver, Doppler shifts for the listed GPS satellites, ephemeris data for each of the listed GPS satellites, and clock correction data for each of the listed GPS satellites. Eliminating the need for the GPS receiver to demodulate the GPS satellite signals also allows the GPS receiver to integrate the GPS satellite signals over a longer period of time allowing for reception of weakened signals due to obstructions.
In order to calculate the auxiliary information for the GPS receiver, however, the approximate location of the GPS receiver must be known. Moreover, the closer the actual location of the GPS receiver to the approximate location used in calculating the auxiliary information the smaller the resulting location search to be performed by the GPS receiver. For example, it is known that if the GPS receiver is given auxiliary information calculated to a location within a radius of one hundred miles of the actual location of the GPS receiver, the GPS receiver need not measure the actual range to the GPS satellites, but instead, only needs to measure a fraction of a millisecond for each of the ranges. This greatly simplifies the necessary range measurements to that of finding the relative code shift position locations to the one millisecond code cycle. In order to do this, however, the GPS receiver still must search all one thousand twenty three code shift positions for all the GPS satellites to be used in the location solution.
The code shift searches can be performed by means of a combination of a fast Fourier transform and an inverse fast Fourier transform correlator to simultaneously search all the code shift positions. This technique for finding the code shift position of a cyclic sequence is described in textbooks, such as Digital Signal Processing by Oppenheim & Shafer. While such an approach is more computationally efficient than a straight correlation, it is nonetheless computationally intensive requiring additional functionality and consuming limited battery power resources. Furthermore, with the opportunity to convey information to the mobile unit to assist it in its search for the GPS satellite ranges, this method then becomes computationally inefficient as it consumes computation cycles searching for many code shift positions that are not possible.
Another solution to searching all one thousand twenty three shift code positions is to build specific hardware to search multiple code shift positions simultaneously. To date, however, hardware specific solutions have not been able to simultaneously search more than a fraction of code shift positions thus requiring multiple searches and lengthy time delays.
In the co-pending, co-assigned U.S. patent application Ser. No. 08/950690, entitled: Reduced Global Positioning System Receiver Code Shift Search Space for a Cellular Telephone System, filed Oct. 15, 1997, a method for providing a reduced functionality GPS receiver residing within a mobile station uses auxiliary information calculated for a known geographical location within a cell serving the mobile station. A server connected to a cellular telephone network calculates auxiliary information based upon a known location within the cell served by the cellular telephone base station. In one instance, the auxiliary information includes a list of GPS satellites in view of the base station, Doppler corrections for each of the listed GPS satellites, and code shift positions for each of the listed GPS satellites based upon a universal time coordinated time for the known location. In another instance, the auxiliary information includes a list GPS satellites in view of the base station, location of the center of coverage of the base station, and locations and clock corrections of the listed satellites based upon a universal time coordinated time.
A drawback to this method is that it depends on time synchronization between the GPS satellites and ranging measurements made by the GPS receiver. In many instances, however, the time at which the GPS receiver makes the range calculations varies due to latency in the cellular telephone network. It would be advantageous, therefore, to devise a method for performing range calculations based on auxiliary information absent time synchronization with the GPS satellites.